add black workflow back

.github/workflows/black.yml

@@ -0,0 +1,21 @@
+name: Black
+
+on: [push, pull_request]
+
+jobs:
+
+  black:
+    # We want to run on external PRs, but not on our own internal PRs as they'll be run
+    # by the push to the branch. Without this if check, checks are duplicated since
+    # internal PRs match both the push and pull_request events.
+    if:
+      github.event_name == 'push' || github.event.pull_request.head.repo.full_name !=
+      github.repository
+
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+      - uses: psf/black@stable
+        with:
+          args: ". --check"